IDL compiler: [LegacyImplementedInBaseClass]

For generated bindings, we already partly handle [LegacyImplementedInBaseClass]
in dependency resolution, but there's one twist: there's no separate
header for these.

This adds a test to compute_dependencies.py to omit this header.

BTW, new test file numbered as *3* b/c I'm doing [ImplementedAs]
in a separate CL, and that's more basic, but a little subtle,
so will post it next.

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/131193012

git-svn-id: svn://svn.chromium.org/blink/trunk@165425 bbb929c8-8fbe-4397-9dbb-9b2b20218538